The instant invention relates to chemical vapor deposition (CVD) apparatus and methods, and more particularly to a parylene deposition apparatus including a post-pyrolysis chamber for capturing unpyrolyzed dimer prior to entry into the deposition chamber, and further including a filter assembly disposed at the inlet of the deposition chamber to filter out sub-microscopic particles and impurities prior to deposition onto a substrate.
Parylene is a general term used to describe a class of poly-p-xylylenes which are derived from a dimer having the structure: ##STR1## wherein X is typically a hydrogen, or a halogen. The most commonly used forms of parylene dimers include the following: ##STR2##
Parylene coatings are obtained from their related parylene dimers by means of a well-known vapor deposition process in which the dimer is vaporized, pyrolized, i.e. cleaved into a monomer vapor form, and fed to a deposition chamber wherein the monomer molecules deposit and polymerize onto a substrate disposed within the deposition chamber. The process occurs according to the following reaction: ##STR3##
Due to their ability to provide thin films and conform to substrates of varied geometric shapes, parylene polymers are ideally suited for use as a conformal external coating in a wide variety of fields, such as for example, in the electronics, automotive, and medical industries.
Octafluoro-[2,2]paracyclophane (Parylene AF4 dimer) is a fluorine substituted version of the above-noted parylene dimers and has the structure: ##STR4## It is known that parylene coatings which are derived from the AF4 dimer by the vapor deposition process have a very high melting temperature (about 500.degree. C.) and a very low dielectric constant (about 2.3). These characteristics make Parylene AF4 ideally suited for many high temperature applications, including, electronic applications, and potentially as an inter-layer dielectric material in the production of semiconductor chips. The existing parylene coating systems as used with Parylene C, D, and N, typically include a multi-chamber vacuum system comprising a vaporization chamber, a pyrolysis chamber coupled to the vaporization chamber, and a deposition chamber coupled to the pyrolysis chamber in which the monomer vapor deposits onto a substrate and polymerizes. The coating systems further include a vacuum system coupled to the chambers for creating sub-atmospheric pressure conditions throughout the chamber system. While the existing parylene deposition systems are highly effective in depositing parylene polymers as an external coating on a variety of articles, there are unique characteristics of semiconductor wafers which prevent the existing parylene coating systems from providing sufficient coating purity to be compatible with existing semiconductor chip manufacturing technologies.
One aspect to semiconductor chip technology is that the environment surrounding the chip wafers during manufacture is normally defined as a clean-room setting wherein virtually all airborne particles are filtered from the environment. Due to the ever decreasing sizes of trace lines and circuit structures, the presence of even a small dust particle, or chemical impurity, on the surface of a wafer is disastrous. The problem with conventional parylene deposition systems is that the parylene vaporization and pyrolysis processes are inherently micro dirty processes, i.e. the reactive monomer vapor is normally laden with foreign airborne particles, and unpyrolyzed dimer which has escaped the pyrolysis chamber. Such impurities have heretofore not been considered a problem, since most parylene coatings are utilized in an external protective coating.
Accordingly, there is currently presented a need for a parylene deposition system particularly suited to providing a highly pure parylene AF4 reactive monomer for deposition onto semiconductor wafers. In this regard, the instant invention provides a parylene deposition system comprising a vaporization chamber, a pyrolysis chamber, a post-pyrolysis chamber for capturing unpyrolyzed dimer prior to entry into the deposition chamber, a deposition bell having a frusto-conical shape to minimize chamber volume and maximize vapor flow, a filter structure positioned at the deposition chamber inlet to filter out microscopic particles and impurities prior to deposition onto the wafer surface, a platen assembly for supporting a semiconductor wafer, an electrostatic clamping device for clamping the wafer in intimate thermal contact with the platen, and a vacuum system for creating sub-atmospheric conditions within the multi-chamber vacuum system.
The vaporization chamber, pyrolysis chamber, post-pyrolysis chamber, and vacuum system, are located within a rectangular housing structure. The platen assembly is preferably located on a top surface of the housing to facilitate placement and removal of the wafers on the platen assembly. The deposition bell is received and secured over the platen assembly to form a deposition chamber. The deposition bell and its associated inlet and outlet pipes and are formed as a single removable unit, and are arranged so as to provide a mating engagement of the deposition bell with the top of the platen and a mating engagement of the inlet and outlet pipes with corresponding fittings on the top of the housing. The removable deposition bell further facilitates access to the platen assembly for the placement and removal of wafers.
The post pyrolysis chamber comprises a cylindrical housing having a plurality of baffle elements disposed in the interior thereof. The reactive monomer exiting from the pyrolysis chamber passes through the post-pyrolysis chamber on its way to the deposition chamber. The post-pyrolysis chamber is maintained at a predetermined temperature conducive to the deposition of the unpyrolyzed dimer, whereby the post-pyrolysis chamber is effective for capturing any unpyrolyzed dimer that may exit the pyrolysis chamber. The outlet of the post-pyrolysis chamber terminates in a fitting located on the top of the housing, and is coupled to the inlet pipe of the deposition bell. The filter structure is disposed adjacent to the inlet end of the deposition bell and preferably comprises a sheet of PTFE filter material captured between two complementary rings which are secured to the inner walls of the deposition bell. The PTFE material is fine enough to effectively filter out sub-microscopic particles while still allowing a sufficient vapor flow through the system, i.e. without the creation of a significant pressure drop across the filter. The combination of the post-pyrolysis chamber and the filter structure is believed to be sufficient to provide the required coating purity for semiconductor wafers.
Accordingly, among the objects of the instant invention are: the provision of a parylene deposition apparatus effective for clean and efficient deposition of Parylene AF4 onto silicon wafers in the production of semiconductor chips; the provision of a parylene deposition apparatus including a post-pyrolysis chamber maintained at a predetermined temperature to effectively capture unpyrolyzed dimer before entry into the deposition chamber; the provision of a parylene deposition apparatus including a filter structure positioned at the deposition chamber inlet to filter out microscopic particles and impurities prior to deposition onto the wafer surface; and the further provision of means for fast, efficient, clean and cost effective deposition of Parylene Af4 onto the surface of a silicon wafer.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.